In the aforementioned example of practical application of the invention, that of large metal tanks and ships, small vehicles are used for examination of their structure, complementary facilities, etc., capable of being displaced vertically over said surface, duly powered, using as means of support against the force of gravity wheels which are also metallic, generally assisted by electromagnets of adequate strength, so that the wheels themselves become a magnet which maintains them adhering adequately to the metallic surface over which they have to be displaced, with the possibility of rotating.
These wheels present a problem which is centred fundamentally on three aspects:                Their metallic nature means that the coefficient of friction with respect to the also metallic surface over which they have to be displaced is minimum, which requires the use of high-powered electromagnets to assure their adherence, which is directly proportional to the volume of their windings, and consequently to the weight of the same, which impacts negatively on the vehicle which is compelled to displace a greater load. This occurrence is specially important in vertical displacements.        The rigidity of the tread surface, arising also from its metallic nature, produces problems in the displacement of the vehicle, specifically when the latter encounters welds or any other type of irregularity which are habitually present on the walls of the tank or element over which said vehicle must move, resulting in loss of adherence and not allowing them to be surpassed.        The dependence on a power supply source, specifically on the battery of the vehicle, signifies a risk from the safety point of view, as well as a substantial consumption of energy.        